You’ve been asking, and hoping it wasn’t the case, but it is. The Tamagotchi On will have a backplate that features a screw, which is one of the few differences between this version and the Japanese Tamagotchi Meets. Bandai America answered this question on the Amazon product page in the Q&A section. They’re not just putting a screw on backplate to annoy you, they have to abide by laws in the United States for safety reasons.
Bandai America goes on to explain that this is actually a mandatory safety mechanism to prevent young kids from taking apart the device, and swallowing the battery. Also, did you know that the screw has to meet safety requirements as well? The screw cannot fall off the backplate when opened, or else this would also be a choking hazard.
This is disappointing for some Tamagotchi fans because historically the quality of the screws has not been the best. With the batteries needing to be changed often, the screws are more susceptible to being stripped and really causing a headache.